Processing equipment in the semiconductor industry is used to make devices whose critical elements have submicron dimensions. Routine maintenance and cleaning is required to keep equipment operating properly which prevents defects in devices processed using that equipment. The current invention relates to the removal of accumulated films from semiconductor processing equipment.
Thin film deposition equipment, and to a lesser extent dry etchers, are notorious for accumulating very thick films including silicon and various metals (Ti, Mo, W, etc.) on the walls of the processing chamber and on internal parts, such as wafer carders, wafer transport mechanisms, and wafer platens, all of which are usually made of anodized aluminum, stainless steel, or quartz (SiO.sub.2). A desired goal is to "clean" the chamber by removing accumulated films from the chamber walls and internal parts without attacking the material of the chamber and those internal parts. This requires that the "selectivity", which is the ratio of the etch rate of the accumulated film to the etch rate of the chamber or internal parts, be high during the cleaning process.
One prior an attempt to clean the processing chamber has included disassembling each piece of equipment and cleaning all of the pans, individually. Such a process has many inherent problems, including a large amount of time needed to disassemble the machine, clean the pans, and reassemble the machine, as well as the ever present possibility that pans become broken or misplaced during this process. Much of the equipment used in semiconductor processing requires that the wafers be processed under vacuum pressure, thus the equipment must be leaked checked after reassembly. If the machine fails its leak check, the leak in the equipment must be found which can take several hours to find. Furthermore, hairline fractures in a piece of equipment may take days to discover which is quite time consuming and very difficult.
Disassembler parts have been cleaned in one of two different methods. The first method has involved scrubbing each component part with an abrasive material such as steel wool or a Scotchbrite.TM. pad to remove accumulated films from the parts. This process of physically removing the accumulated film is inherently particle ridden, and therefore, these parts must be cleaned outside the fabricating area or moved to a hood or other vented portion of the fabricating area so that particles are not in the same processing areas as semiconductor wafers. If a hood or other vented areas are used within the fabricating area, costly manufacturing floor space is lost. Particles generated from this abrasive cleaning process may also be a safety hazard in that loose particles on a floor may cause people to slip and fall or if the particles are airborne, they could get into someone's eyes or become ingested.
The abrasive methods mention above have additional drawbacks, including destroying or altering delicate critical parts of the system. These methods could remove substantial amounts of the chamber material thereby requiring that the chamber walls or the internal parts be replaced sooner than normal. Another problem is that the abrasive methods scratch or make indentations in the chamber walls or internal parts which provide a location where particles accumulate. During wafer processing after the machine is reassembled, these particles could actually wind up on the wafers causing wafer yields to decrease. In extreme cases where the amount of material removed is great, the time needed to evacuate the system to a baseline pressure may take noticeably longer which adds further delays to a procedure that is already very long.
As an alternative to the mechanical methods described above, the disassembled parts could be cleaned in an acid bath with acids including HF, H.sub.2 SO.sub.4, H.sub.3 PO.sub.4, and HNO.sub.3. While these acids react with the accumulated film to be removed, they also react with the materials that make up the chamber walls and internal parts. As an example, consider polysilicon which deposits on the walls of the quartz tube (SiO.sub.2) during a typical polysilicon deposition process. To clean the quartz tube, a combination of HF and HNO.sub.3 is used to etch the polysilicon, wherein, HNO.sub.3 reacts with the polysilicon to from SiO.sub.2 which is etched away by HF. Unfortunately, HF also etches the quartz tube which is the processing chamber for the polysilicon deposition. It is desired to etch away the polysilicon without etching any of the quartz tube or only a minimal amount of that quartz tube. HF removes the SiO.sub.2 from outside of the quartz tube as well as the SiO.sub.2 from polysilicon that is oxidized by the HNO.sub.3. For example, a 20 .mu.m deposition of polysilicon within a quartz tube needs to be removed. The HNO.sub.3 converts the 20 .mu.m film polysilicon film into more than 40 .mu.m of SiO.sub.2. The HF removes the more than 40 .mu.m of SiO.sub.2 of the converted polysilicon film as well as more than 40 .mu.m of the outer tube wall since the tube is submerged in acid. Wet chemical methods involving acids have another drawback in that many metals corrode in the presence of acid severely restricting the use of acids within most processing equipment.
A further problem with wet chemical methods involves pumping down the reactor after being reassembled. The acids are normally rinsed away with water. If water remains in joints or crevices of the equipment during its first pump down, the water turns to ice which exerts great force on the joints and crevices of the equipment and could damage or destroy expensive processing equipment.
Yet another problem with disassembling the system is that water from air can be adsorbed onto the walls of the chamber and onto the surfaces of the internal pans. The water adsorbed onto the walls and the surfaces is difficult to outgas. The initial pumpdown following the reassembling may take several hours to reach a baseline pressure, thereby causing further delays in addition to the time for disassembly and reassembly as previously addressed. Also, it is possible that a long outgas period may be mistaken as a leak in the system. An agonizing leak check will be perform that does not indicate any leaks, and therefore, several man-hours will be wasted trying to find a leak that does not exist.
Another prior an attempt involves the use of plasmas which are ionized gases that react with the accumulated film while slowly reacting with the materials that make up the processing chamber and its parts. A major problem exists with plasmas in that many have etching selectivities in the range of 10:1 or less, which means that the plasma etches process walls at a rate of 10% or more of the etch rate of the accumulated film.
Use of a plasma etch suffers from additional detriments including the plasma being directed by a radio frequency (RF) field. The plasma cannot reach the cracks and crevices or more remote locations of the reactor. Thus, the accumulated film is removed from within a very specifically defined region, but rarely does it get removed from the entire chamber at one time. Either remote locations are not etched, or the removal takes several steps. For example, with polysilicon deposition systems, there is a plasma process called a Benzing process where the cantilever or other loading mechanism for the polysilicon deposition system is removed. The plasma lance is inserted into the tube and aligned which becomes more difficult as the tube length increases. After alignment, the endplate of the lance seals the tube, the tube is pumped down, allowed to stabilized at a certain pressure, NF.sub.3 flows into the tube, and a plasma is struck using the RF field generated by the electrodes within the plasma lance. One problem with this approach is that NF.sub.3 is highly toxic and its vapors must be controlled down to a level of less than 2500 PPM. In addition, the plasma is only going to be within a specific area, which for the polysilicon tube is the main deposition zone and would not include the bell-jar portion of the tube or the areas adjacent to the door flange. Once the Benzing process is finished, the cantilever must be reinstalled and aligned. The realignment of the cantilever is highly critical since chemical vapor depositions, such as a polysilicon deposition, are extremely dependent on the wafer location within the chamber.
Another prior art attempt to remove polysilicon uses high temperature HCl gas which is highly corrosive and should be avoided. Ideally, no etching system should require disassembling the machine, having to strike a plasma, or using highly corrosive materials such as HCl. One of the few compounds which can achieve these objectives is xenon fluoride (XeF.sub.2). XeF.sub.2 is very expensive making its use prohibitive for normal commercial applications where frequent etchings are required and has a low vapor pressure making its use in some applications very difficult.